In pump assemblies which comprise one or more impellers driven by a prime mover connected to the impeller by a drive shaft, mechanical seal assemblies are generally provided for preventing substantial fluid leakage along the drive shaft. These seal assemblies may include multiple mechanical seals, each comprising rotatable and stationary seal rings each having a seal face opposing one another, the rotatable seal ring being connected to the drive shaft or a drive shaft sleeve while the stationary seal ring is connected to a housing or pump case. In some of the multiple units, as described, the second seal assembly acts as a back-up for the first assembly. In the event of failure of both seal assemblies of a multiple unit, pumped fluid can leak along the shaft and escape from the housing.
It is also known in the art to construct pumps with primary and secondary mechanical seals, as for example, the arrangement described in Biheller U.S. Pat. No. 3,096,985. The secondary seal in that reference comprises a sleeve member having a serrated edge which engages a tapered section of a drive shaft and which acts as a labyrinth to break down the pressure of the leaking fluid to either eliminate or severely limit leakage along the shaft to the exterior of the housing. There is still a need for a non-engaging and thus non-wearing secondary seal which effectively prevents fluid leakage when the primary seal fails.